Bismuth Doped TiO2 Functionalized Hetero-Junction Semiconductor Anode with an Enhanced Reactive Chlorine Generation for Use in Electrochemical Wastewater Treatment

An electrolysis cell with a high-salinity wastewater as electrolyte is capable of anodic transformation of recalcitrant pollutants. In particular, chloride in the wastewater, which provides the ionic strength, also mediates an indirect oxidation of organic or inorganic compounds by producing reactive chlorine species (RCS). To this end, one can imagine that an enhanced RCS generation can reduce the energy consumption and hydraulic retention time. Metal oxide anodes, the core of electrochemical wastewater treatment system, have been traditionally categorized depending on an existence of electrocatalytic activity for oxygen evolution reaction (OER). PbO₂ are SnO₂ are well known materials with relatively high current efficiency (selectivity) toward heterogeneous oxidation of pollutants or chloride by formation of surface-bound hydroxyl radical. However, a large kinetic barrier frequently observed for these fully-oxidized metal oxides require a large energy consumption to increase the reaction rate (current). On the other hand, Pt group metal oxides (IrO₂ and RuO₂) with low OER overpotentials brought about a prodigious progress in water splitting and chloro-alkali cell. Redox transition of electroactive materials readily quenches the surface hydroxyl radicals to form higher oxides, resulting in a greater current response but with lower current efficiency toward the oxidation of chloride. In this presentation, we report a hetero-junction semiconductor anode with an enhanced RCS generation. The outer surface of our novel anodes is functionalized with bismuth-doped TiO₂ which provide the surface hydroxyl group, while underlying IrO₂/Ta₂O₅ layer serves as an electron shuttle.  

The hetero-junction semiconductor anodes were prepared by sequential thermal decomposition method. H₂IrCl₆ and TaCl₅ were utilized as precursors for the IrO₂/Ta₂O₅ (Ir:Ta = 7:3) layer annealed at 525 ºC. Aqueous titanium-glycolate complex was prepared from either hydroxo-peroxo titanium or titanic acid, which was mixed with bismuth citrate solution with variable molar ratio of Ti to Bi (annealing at 425 ºC). Prepared electrodes were analyzed by SEM, XRD, FT-IR and XPS techniques. Electroanalytic experiments including cyclic voltammetry (CV) and Tafel analysis were performed to assess the OER onset-potential and current response under variable anodic potentials. The current efficiencies of reactive chlorine generation were estimated using DPD reagent under potentiostatic conditions (2~3 V versus NHE) in dilute chloride solutions (50 mM NaCl) with circum-neutral pH. In addition, the stability of the electrodes was assessed by an accelerated life test.

The Bi-doped TiO₂ in the form of transparent thin film showed negligible current generation without the underlying IrO₂/Ta₂O₅. Comparing CV data of the hetero-junction anode with IrO₂/Ta₂O₅ anode indicated that the over-coating of Bi-doped TiO₂ increases the OER onset-potential by 100~200 mV depending on the molar ratio of Ti to Bi, which was in consistent with the potentiostatic current densities reduced by 15 ~ 50%. Nevertheless, the hetero-junction fabrication was always found to increase the current efficiency of RCS generation. The highest current efficiency was observed for the Ti:Bi ratio of 7:3, which approached virtually unity (100%). A series of material characterization and electro-analysis suggested that the hydration of TiO₂ provides active sites for the hydroxyl radical formation, while partial substitution with Bi resulted in a distortion in the crystalline structure and an increase in surface area. Consequently, the potentiostatic generation rate of reactive chlorine for Bi_0.3Ti_0.7O_x anode was almost twice of that for IrO₂/Ta₂O₅. The accelerated life test predicted the lifetime of the hetero-junction anodes to be more than 2 year under normal operational range (300 A m^-2), thereafter a dissolution of Ir was observed. Therefore, our hetero-junction fabrication can also elongate the service life without a loss of precious element.